$prefix:todo-;
@mixin setProperty($prop,$value,$unit: null){
    @if $unit {
        #{$prop}:$value + $unit; 
    }@else{
        #{$prop}:$value; 
    }
}
$white:#ffffff;
$black:#000000;
.flex-center {
    @include setProperty(display,flex);
    @include setProperty(justify-content,center);
    @include setProperty(align-items,center);
}
.reset {
    @include setProperty(margin,0);
    @include setProperty(padding,0);
    @include setProperty(box-sizing,border-box);
}